It was as good a Patriots Day, as good a Marathon day, as any — dry and seasonably warm but not hot like last year. The buzz was great. While the runners climbed Heartbreak Hill, the Red Sox were locked in another white-knuckle duel with the Tampa Bay Rays at Fenway Park. The only thing missing was Lou Reed crooning “Perfect Day” in the background.

The winners and the elite runners had long ago finished, when in the Fens, at shortly after 2 p.m., Mike Napoli kissed a ball off The Green Monster in the bottom of the ninth, allowing Dustin Pedroia to scamper all the way home from first base, giving the Red Sox a walk-off win.

Many of those jubilant Sox fans had walked down through Kenmore Square toward the Back Bay to watch the Marathon. Some of them had just got to the finish line when the first bomb went off, shortly before 3 p.m.

In an instant, a perfect day had morphed into something viscerally evil.

The location and timing of the bombs were sinister beyond belief, done purposely to maximize death and destruction. Among those who watched in horror as a fireball belched out across the sidewalk on Boylston were the parents of the school kids murdered in Newtown, Conn. The Atlantic reported they were sitting in a VIP section at the finish line, across the street from the explosion.

It would be wrong and a cliche to say we lost our innocence on Monday afternoon as a plume of white smoke drifted high above Boylston Street, as blood pooled on the sidewalk across from the Boston Public Library, as severed limbs lay amid the bruised and the bloodied and the stunned, their ears ringing, their ears bleeding.

We lost our innocence on another perfect day, in September, 12 years ago. But we lost something Monday, too, and that is the idea that we will ever feel totally safe in this city again.

The Marathon is the city’s signature event, a tangible link with the rest of the world. It is one of the few things that allows us to cling to that pretense of Boston as the Hub of the universe. Patriots Day is a celebration of our revolutionary history, but we share it with the world. It is the one day of the year when the city is its most diverse, with people from so many other countries here to run those 26 miles from Hopkinton to the Back Bay.

And so it was alternately poignant and horrifying to watch as first responders frantically pulled metal barriers and the flags of so many different countries down into Boylston Street in a desperate rush to get to the dead and the injured on the sidewalk.

Those flags looked like victims, splayed on Boylston Street as acrid smoke hung in the air.

After the initial explosion, runners instinctively craned their necks toward the blast site. Then, 12 seconds later, a second explosion, further up Boylston. It was pandemonium. I saw an older runner wearing high rise pink socks, about to cross the finish line. He was knocked to the ground by a photographer running up Boylston Street toward the second explosion.

In an instant, so many lives changed. Some ended. The telephone lines burned. Everybody was trying to figure out who and why. The cops I talked to were shaking their heads. It could be anybody. Could be foreign. Could be domestic. Could be al-Qaida. Could be home-grown nuts. It was Patriots Day. It was tax day. It was Israel’s independence day. Theories swirled like the smoke above Boylston Street. Friday marks the 20th anniversary of the FBI assault on the Branch Davidian compound in Waco, Texas, and the 18th anniversary of the Oklahoma City bombing.

Then there was the story about the young Saudi guy who was being questioned by the FBI. Now, the FBI wouldn’t tell me if my pants were on fire, but my old pal John Miller from CBS News reported that the kid did a runner after the explosion and that somebody tackled him and held him for the police. Miller used to be an associate director at the FBI, and let’s just say his sources there are impeccable. Miller says the Saudi guy was cooperative and denied he had anything to do with the bombing. He says he took off because, like everybody else in the Back Bay, he was terrified. A law enforcement source later told me that Miller’s story is right on the money.

I saw Lisa Hughes from WBZ-TV trying to do her job, amid the blood and the body parts. And then I remembered that Lisa, who is as nice a person as you’ll find in this business, married a guy from Wellesley named Mike Casey who lost his wife Neilie on one of the planes out of Boston that crashed into the Twin Towers. And then I tried not to cry and just marveled at how professional Lisa was.

Massachusetts Gov. Deval Patrick began his day by visiting ailing Mayor Tom Menino of Boston in the hospital. Hours later, Patrick was on the phone with President Obama, and Menino signed himself out of the hospital. He couldn’t be cooped up while his city was being attacked. Like so many people in the Back Bay, the mayor needed a wheelchair to get around.

Dave McGillivray, the Marathon director, had just arrived in Hopkinton, and was about to run the 26-mile route, as he does every year hours after the last runner has departed. A state cop told McGillivray what had happened and McGillivray jumped in a cruiser and raced back to the finish line.

Before 3 p.m., the medical tent at the finish line had seen nothing worse than a blister. Then, in an instant, it was transformed into a battlefield triage unit. Doctors and nurses who had been running the race in turn raced to the medical tent and volunteered their services, still sweating, still wearing their running gear. People in the Back Bay opened their homes to runners who couldn’t get back to their hotels.

We will get through this, but we will never be the same.

Even as the smoke drifted away from Boylston, we are still in the fog, still in the dark, our ears still ringing from the bombs.

And we are left with this unnerving proposition: If it was home-grown, it was probably an aberration, the work of a lunatic. If it was foreign inspired or sponsored, we will never feel safe again in our own town.

President Obama asked the rest of the country to pray for Boston. But we need more than prayers. We need answers. We need peace of mind, and we’ll never have that again on Patriots Day. Ever. Because somebody came here on our Patriots Day and launched their own revolution.